The present invention relates to cardiac pacing systems and methods, and, more particularly, to cardiac pacing systems, which provide for the measurement and processing of data related to the source impedance of electrical cardiac signals.
In the introduction it should be clearly pointed out that all the prior art referred to related to the stimulus-impedance, i.e. the impedance that the tissue represents during the delivery of a (subthreshold) stimulus. Our invention relates to the source impedance of cardiac signals, either spontaneous ones or evoked ones.
Implanted cardiac pacemakers are employed to assist patients suffering from severe bradycardia or chronotropic incompetence. Originally, such pacemakers restored a normal, at rest, heart rate by providing either a fixed rate or a narrow range of externally programmable rates. However, these pacemakers failed to meet patients"" metabolic demands during exercise. Consequently, so-called xe2x80x9crate adaptivexe2x80x9d or xe2x80x9crate responsivexe2x80x9d pacemakers were developed. These pacemakers sense one or more parameters correlated to physiologic need and adjust the pacing rate of the pacemaker accordingly.
One way to sense the above-stated parameter is to measure the impedance between a pacing electrodes. Various pulse-based impedance sensors have been proposed or are now in use with cardiac stimulators for deriving both hemodynamic and other physiologic parameters. Generally, these sensors deliver trains of fairly low-energy probing pulses between two or more electrodes of a pacing or defibrillation lead system. Each train contains pulses delivered at the rate of between 1 and 500 per second. In general, these pulses have a biphasic morphology in order to balance the charge delivered to the tissue, thus avoiding ion migration and electrolysis within the living tissue, as well as reducing interference on external monitoring equipment. In addition, charge balancing reduces the possibility of actually capturing the heart muscle by these low energy pulses with low-threshold leads.
However, the prior art has not developed an effective means by which to measure the source impedance of the cardiac electrical signals. In fact, the prior art referred to herein relates to the stimulus-impedance of the heart; i.e., the impedance that the tissue represents during the delivery of a (subthreshold) stimulusxe2x80x94not to the source impedance of cardiac signals, either spontaneous or evoked ones. For example, in Renirie et al., U.S. Pat. No. 3,989,958, there is provided an amplifier circuit for detecting low level signals of positive or negative amplitude, and for producing an output pulse whenever the input signal exceeds a predetermined threshold level. Although Renirie provides an example of an operational transconductance amplifier (OTA) sense amplifier, Renidie does not provide a means with which to sense the voltage (open source through a high input impedance amplifier) and the current (short-circuited source through low xe2x80x98zeroxe2x80x99 input impedance amplifier) of the cardiac electrical signal.
Similarly, Renirie et al., U.S. Pat. No. 3,989,959, Renirie, U.S. Pat. No. 4,023,046, and Renirie, U.S. Pat. No. 4,097,766, all likewise provide examples of OTA (current-based) sense amplifiers.
In Thompson et al., U.S. Pat. No. 4,275,737, there is disclosed a cardiac pacemaker responsive to a natural heart activity for affecting the operation of the pacemaker. Thompson discloses an example of a high impedance sense amplifier. However, Thompson does not disclose the use of a current sensing amplifier.
Examples of minute volume-based rate responsive pacemakers utilizing a cardiac impedance measurement consisting of simultaneous current stimulus and voltage monitoring mechanisms are disclosed in Nappholz et al., U.S. Pat. No. 4,702,253, Aft, U.S. Pat. No. 4,884,576, and Yedch et al., U.S. Pat. No. 5,562,711. However, these patents measure the pacing impedance of the applied low-energy pulses and do not involve the simplified improvement of the present invention; that is, the measurement of a source impedance of cardiac signals based upon a voltage signal and a current signal.
In Hudrlik, U.S. Pat. No. 5,282,840, there is disclosed a physiological monitoring system for monitoring the condition of a patient""s body tissue. Hudrlik discloses an impedance measurement system that uses a field density clamp for detecting ischemia as well as drug monitoring and titration.
In Kroll, U.S. Pat. No. 5,431,687, there is disclosed another implantable cardiac defibrillator. Kroll discloses another example of cardiac impedance sensing which senses the impedance of the heart during stimulus delivery.
Buldino, U.S. Pat. No. 5,507,785, and Prutchiet al., U.S. Pat. No. 5,713,935 disclose subthreshold biphasic current stimulus systems used to determine cardiac impedance during these stimuli. In Buldino and Prutchi, the cardiac impedance sensed is used for rate response; that is, for a minute volume and stroke volume, detection mechanisms.
Finally, in Panescu et al., U.S. Pat. No. 5,577,509, there is disclosed a method for examining heart tissue morphology using a pair of electrodes, at least one of which is located in contact with the heart tissue. Panescu examines the cardiac tissue through current-driven impedance measurements circuits. Additionally, Panescu uses multiplexing to facilitate data processing.
As discussed above, the most pertinent prior art patents are shown in the following table:
All the patents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Preferred Embodiments and the Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
The present invention is therefore directed to providing a method and system for measuring and processing data related to the source impedance of at least one cardiac electrical signal in a mammalian heart. Such a system of the present invention adds important information to cardiac pacing systems related to the condition of the cardiac muscle and the electrical signals that it produces, currently not available in cardiac pacing systems.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art respecting the measurement of a source impedance of a cardiac electrical signal of a mammalian heart. Those problems include, without limitation: the ability to accurately measure both current and voltage with two different amplifier systems, the ability to switch between the two different amplifier systems and to process and store the data that was obtained for either immediate or later processing. Unlike the prior art above, the present invention relates to the stimulus-impedance (i.e., the impedance that the tissue represents during the delivery of a (subthreshold) stimulus), rather than the source impedance of spontaneous or evoked signals.
In comparison to known techniques for measuring cardiac parameters such as ischemia, the repolarization time period, various embodiments of the present invention may provide the following advantage, inter alia, i.e., the measurement of the source impedance of a cardiac electrical signal and switching between two differing amplifier systems.
Some of the embodiments of the present invention include one or more of the following features: an implantable medical device including at least one sensing lead, at least one pacing lead, a microprocessor and an input/output circuit including a digital controller/timer circuit, an output amplifier, a voltage sense amplifier, a current sense amplifier, a peak sense and threshold measurement device and a comparator. Furthermore, in accordance with the resent invention, a method of measuring a source impedance of at least one cardiac electrical signal in a mammalian heart is provided. A first amplifier system, e.g. the voltage sensing amplifier is operated. A first signal from a chamber of the heart is received. The first signal is passed through a first amplifier. A second amplifier system, e.g. the current sensing amplifier, is then operated. A second signal from the chamber of the heart is received. The second signal is passed through a second amplifier. Finally, the source impedance of the at least one cardiac electrical signal is calculated. The source impedance is based on the amplified first signal and the amplified second signal. The switching frequency between the two different amplifiers ranges from several kiloHerz, providing an accurate source impedance signal of every single electrical cardiac signal to once every beat, second up to minutes, providing a more averaged trend of the source impedance signal of the cardiac chamber.